The long-term goals of this research are to determine the mechanisms by which polychlorinated biphenyls (PCBs) disrupt thyroid hormone (TH) action during brain development, to define the neurological consequences of this disruption in rodent model systems, and to translate this information to study human populations. PCBs are widespread and persistent environmental contaminants, and incidental exposure to PCBs has been associated with reduced TH levels in pregnant women, lower birth weight and early growth rate, and neurological deficits. We propose that a combination of coplanar (dioxin-like) and non-coplanar PCBs are required to produce metabolites that bind to the TH receptor (TR), producing effects that are dependent on the TH response element (TRE), cellular context and TR isoform. The resulting effects on brain development are not predictable based on our current knowledge. To test this hypothesis and its implications, we will identify specific PCB metabolites in cell culture and in animals following treatments with defined mixtures. AhR-null and CYP1A1-null mice will be employed to confirm the role of AhR and CYP in the generation of PCB metabolites and effects on TH signaling. Specific metabolites will be tested for their ability to bind to rat and human TR11 and TR21 isoforms. Metabolites that exhibit binding will be characterized for their ability to interfere with TH signaling in selected cells in culture. In vitro studies will specifically address the ability of PCB metabolites to act as thyroid hormone agonists or antagonists. A combination of approaches including luciferase reporter assays, electrophoretic mobility shift (EMSA) and chromatin immunoprecipitation (ChIP) will be use in these in vitro studies. Human cells lines derived from liver, fibroblast, monocytes and neurons will be used to test whether these molecular events occur in humans. Endpoints of TH actions disrupted by PCBs in human fibroblasts or monocytes may be useful in studying these events in human populations.